Oil and gas production wells are often fractured to improve recovery of oil and gas. Such fractures are typically provided by placing a fracturing fluid containing thickener and proppant in the wellbore and then applying a pressure that exceeds the fracture gradient of the formation. The proppants are held in suspension by a high viscosity imparted by the thickener. The fracturing fluid must have properties that prevent fluid loss to the formation in order to prevent the proppant from "sanding out" prior to the fracture reaching an effective size. The high viscosity of the fracturing fluid provide at least a portion of these fluid loss properties.
Polysaccharides are typically used as the polymer thickeners in fracturing fluids. Suitable polysaccharides include galactomannan gums, glucomannan gums, guars, derived guars and cellulose derivatives. Specific examples are guar gum, guar gum derivatives, locust bean gum, caraya gum, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose and hydroxyethyl cellulose.
Polymeric thickeners can be made to be effective at lower concentrations by crosslinking the polymers. Polysaccharides can be crosslinked by metal ions such as aluminum, antimony, borate, zirconium and titanium containing compounds.
After a fracture is formed, and the proppant is in place, it is desirable to have the viscosity of the fracturing fluid quickly decrease so that production from the formation will not be inhibited by the thickened fracturing fluid. If the formation temperature is above about 200.degree. F., the viscosity of the fracturing fluid will decrease because of thermal degradation of the polymer. At temperature less than 200.degree. F., decreasing the viscosity of the fracturing fluid is often accomplished by including a "breaker" in the fracture fluid.
Breakers may be enzymes, oxidants or acids. These typical breakers begin reducing the solution's viscosity immediately, and the amount of polymer required to effectively thicken the fracturing fluid for the required time period is increased when a breaker is required. It would therefore be desirable to delay the viscosity reduction until after the polymer has served it's purpose. Effectively breaking polymers that have been used in fracturing fluids remains a problem, and in particular, effectively breaking polymers at temperatures below about 200.degree. F. is a problem.
Borates are often used as crosslinking agents for polymeric thickeners. Borates are effective as crosslinking agents in basic solutions, and tend to disassociate from the polymers if the solution pH decreases below a pH of about 8. U.S. Pat. No. 5,271,466 discloses a process wherein this tendency to disassociate in acidic environments is utilized to break the polymer gel in a fracturing fluid by first injecting into the fracture a fracturing fluid crosslinked by a crosslinking agent that is effective in an acidic solution, and then injecting a borate crosslinked polymer gel in a basic solution. The two solutions then combine in the formation thereby lowering the pH to which the borate crosslinked polymer is exposed, and causing the borate crosslinks to disassociate. This process remains dependent upon thickened solutions mixing within a fracture, and is therefore less than totally effective.
U.S. Pat. No. 5,067,566 discloses a process wherein a borate crosslinked polymer is used in a fracturing fluid with an enzyme breaker and a pH regulating substance that slowly hydrolyzes to produce a Bronsted acid, thereby dropping the pH of the fracturing fluid. Enzyme breakers are not particularly effective in solutions having a pH above about 8, and the pH is lowered in the process of patent '566 to "activate" the enzyme breaker. Enzyme breakers are sensitive to many variables, and become denatured and inactive at temperatures above about 130.degree. F. They are also relatively expensive.
It is therefore an object of the present invention to provide a method for fracturing a subterranean formation from a wellbore wherein a fracturing fluid is utilized that will, at temperatures between about 120.degree. F. and about 200.degree. F., effectively reduce in viscosity to a viscosity of 10 cp or less within a time period of less than about two days. It is a further object to provide a fracturing fluid useful in such a method.